Electrochemical production of synthesis gas from carbon dioxide

ABSTRACT

A method for electrochemical production of synthesis gas from carbon dioxide is disclosed. The method generally includes steps (A) to (C). Step (A) may bubble the carbon dioxide into a solution of an electrolyte and a catalyst in a divided electrochemical cell. The divided electrochemical cell may include an anode in a first cell compartment and a cathode in a second cell compartment. The cathode generally reduces the carbon dioxide into a plurality of components. Step (B) may establish a molar ratio of the components in the synthesis gas by adjusting at least one of (i) a cathode material and (ii) a surface morphology of the cathode. Step (C) may separate the synthesis gas from the solution.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 61/315,628, filed Mar. 19, 2010, which is hereby incorporated byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates to chemical reduction generally and, moreparticularly, to a method and/or apparatus for implementingelectrochemical production of synthesis gas from carbon dioxide.

BACKGROUND OF THE INVENTION

The combustion of fossil fuels in activities such as electricitygeneration, transportation and manufacturing produces billions of tonsof carbon dioxide annually. Research since the 1970s indicatesincreasing concentrations of carbon dioxide in the atmosphere may beresponsible for altering the Earth's climate, changing the pH of theoceans and other potentially damaging effects. Countries around theworld, including the United States, are seeking ways to mitigateemissions of carbon dioxide.

A mechanism for mitigating emissions is to convert carbon dioxide intoeconomically valuable materials such as fuels and industrial chemicals.If the carbon dioxide is converted using energy from renewable sources,both mitigation of carbon dioxide emissions and conversion of renewableenergy into a chemical form that can be stored for later use will bepossible. Electrochemical and photochemical pathways are techniques forthe carbon dioxide conversion.

Previous work in the field has many limitations, including the stabilityof systems used in the process, the efficiency of systems, theselectivity of the system or process for a desired chemical, the cost ofmaterials used in systems/processes, the ability to control the processeffectively, and the rate at which carbon dioxide is converted. Existingsystems for producing synthesis gas rely on gasification of biomass orsteam reformation of methane. The processes use high temperatures andpressures. In the case of synthesis gas made from fossil fuels, liquidfuels made therefrom increase greenhouse gas emissions. Synthesis gasfrom biomass can reduce greenhouse gas emissions, but can be difficultto convert efficiently and produces unwanted ash and other toxicsubstances. No commercially available solutions for converting carbondioxide to economically valuable fuels or industrial chemicals currentlyexist. Laboratories around the world have attempted for many years touse electrochemistry and/or photochemistry to convert carbon dioxide toeconomically valuable products. Hundreds of publications exist on thesubject, starting with work in the 19th century. Much of the work doneprior to 1999 is summarized in “Greenhouse Gas Carbon Dioxide MitigationScience and Technology”, by Halmann and Steinberg. A more recentoverview of work on electrochemical means of reducing carbon dioxide is“Electrochemical Carbon Dioxide Reduction—Fundamental and Applied Topics(Review)”, by Maria Jitaru in Journal of the University of ChemicalTechnology and Metallurgy, 2007, pages 333-344.

Laboratory electrochemical methods usually involve a small (i.e., <1liter) glass cell containing electrodes and an aqueous solution withsupporting electrolyte in which carbon dioxide is bubbled, though asolvent other than water can be used. Reduction of the carbon dioxidetakes place directly on the cathode or via a mediator in the solutionthat is either a transition metal or a transition metal complex.Photoelectrochemical methods also incorporate aqueous solutions withsupporting electrolyte in which carbon dioxide is bubbled. The maindifference is that some or all of the energy for reducing the carbondioxide comes from sunlight. The reduction of the carbon dioxide takesplace on a photovoltaic material, or on a catalyst photosensitized by adye. All systems developed to date have failed to make commercialsystems for the reasons outlined above. The systems developed inlaboratories could not be scaled to commercial or industrial sizebecause of various performance limitations.

Existing electrochemical and photochemical processes/systems have one ormore of the following problems that prevent commercialization on a largescale. Several processes utilize metals such as ruthenium or gold thatare rare and expensive. In other processes, organic solvents were usedthat made scaling the process difficult because of the costs andavailability of the solvents, such as dimethyl sulfoxide, acetonitrileand propylene carbonate. Copper, silver and gold have been found toreduce carbon dioxide to various products. However, the electrodes arequickly “poisoned” by undesirable reactions on the electrode and oftencease to work in less than an hour. Similarly, gallium-basedsemiconductors reduce carbon dioxide, but rapidly dissolve in water.Many cathodes make a mix of organic products. For instance, copperproduces a mix of gases and liquids including methane, formic acid,ethylene and ethanol. A mix of products makes extraction andpurification of the products costly and can result in undesirable wasteproducts to dispose. Much of the work done to date on carbon dioxidereduction is inefficient because of high electrical potentials utilized,low faradaic yields of desired products and/or high pressure operation.The energy consumed for reducing carbon dioxide thus becomesprohibitive. Many conventional carbon dioxide reduction techniques havevery low rates of reaction. For example, some commercial systems havecurrent densities in excess of 100 milliamperes per centimeter squared(mA/cm²), while rates achieved in the laboratory are orders of magnitudeless.

SUMMARY OF THE INVENTION

The present invention concerns a method for electrochemical productionof synthesis gas from carbon dioxide. The method generally includessteps (A) to (C). Step (A) may bubble the carbon dioxide into a solutionof an electrolyte and a catalyst in a divided electrochemical cell. Thedivided electrochemical cell may include an anode in a first cellcompartment and a cathode in a second cell compartment. The cathodegenerally reduces the carbon dioxide into a plurality of components.Step (B) may establish a molar ratio of the components in the synthesisgas by adjusting at least one of (i) a cathode material and (ii) asurface morphology of the cathode. Step (C) may separate the synthesisgas from the solution.

The objects, features and advantages of the present invention includeproviding a method and/or apparatus for implementing electrochemicalproduction of synthesis gas from carbon dioxide that may provide (i)cathode combinations for simultaneous evolution of carbon monoxide andhydrogen gas using carbon dioxide and water as feedstock, (ii)combinations of cathode materials, electrolytes, electrical potentials,pH levels, carbon dioxide flow rates and/or heterocycle catalysts, usedto get a desired molar ratios of carbon monoxide and hydrogen gas, (iii)specific process conditions that optimize the carbon dioxide conversionto carbon monoxide while optimizing hydrogen gas evolution, (iv) achoice of specific configurations of heterocyclic amine catalysts withengineered functional groups, (v) process conditions that may facilitatelong life electrode and cell cycling and/or (vi) process conditions thatmay provide long-term product recovery.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the presentinvention will be apparent from the following detailed description andthe appended claims and drawings in which:

FIG. 1 is a block diagram of a system in accordance with a preferredembodiment of the present invention;

FIG. 2 is a table illustrating relative organic product yields fordifferent cathodes;

FIG. 3 is a formula of an aromatic heterocyclic amine catalyst;

FIGS. 4-6 are formulae of substituted or unsubstituted aromatic 5-memberheterocyclic amines or 6-member heterocyclic amines;

FIG. 7 is a flow diagram of an example method used in electrochemicalexamples; and

FIG. 8 is a flow diagram of an example method used in photochemicalexamples.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with some embodiments of the present invention, anelectro-catalytic system is provided that generally allows carbondioxide to be converted at modest overpotentials to highly reducedspecies in an aqueous solution. Some embodiments generally relate to anevolution of carbon monoxide and hydrogen gas from carbon dioxide andwater. Carbon-carbon bonds and/or carbon-hydrogen bonds may be formed inthe aqueous solution under mild conditions utilizing a minimum ofenergy. In some embodiments, the energy used by the system may begenerated from an alternative energy source or directly using visiblelight, depending on how the system is implemented.

The reduction of carbon dioxide may be suitably catalyzed by aromaticheterocyclic amines (e.g., pyridine, imidazole and substitutedderivatives.) Simple organic compounds have been found effective andstable homogenous electrocatalysts and photoelectrocatalysts for theaqueous multiple electron, multiple proton reduction of carbon dioxideto organic products, such as formic acid, formaldehyde and methanol.High faradaic yields for the reduced products have generally been foundin both electrochemical and photoelectrochemical systems at low reactionoverpotentials.

Some embodiments of the present invention thus relate to environmentallybeneficial methods for reducing carbon dioxide. The methods generallyinclude electrochemically and/or photoelectrochemically reducing thecarbon dioxide in an aqueous, electrolyte-supported dividedelectrochemical cell that includes an anode (e.g., an inert conductivecounter electrode) in a cell compartment and a conductive or p-typesemiconductor working cathode electrode in another cell compartment. Acatalyst of one or more substituted or unsubstituted aromaticheterocyclic amines may be included to produce a reduced organicproduct. Carbon dioxide may be continuously bubbled through the cathodeelectrolyte solution to saturate the solution.

For electrochemical reductions, the electrode may be a suitableconductive electrode, such as Al, Au, Ag, Cd, C, Co, Cr, Cu, Cu alloys(e.g., brass and bronze), Ga, Hg, In, Ir, Mo, Nb, Ni, Ni alloys, Ni—Fealloys, Os, Pd, Pt, Rh, Ru, Sn, Sn alloys, Ti, V, W, Zn, stainless steel(SS), austenitic steel, ferritic steel, duplex steel, martensitic steel,Nichrome, elgiloy (e.g., Co—Ni—Cr), degenerately doped p-Si,degenerately doped p-Si:As and degenerately doped p-Si:B. Otherconductive electrodes may be implemented to meet the criteria of aparticular application. For photoelectrochemical reductions, theelectrode may be a p-type semiconductor, such as p-GaAs, p-GaP, p-InN,p-InP, p-CdTe, p-GaInP₂ and p-Si. Other semiconductor electrodes may beimplemented to meet the criteria of a particular application.

The catalyst for conversion of carbon dioxide electrochemically orphotoelectrochemically may be a substituted or unsubstituted aromaticheterocyclic amine. Suitable amines are generally heterocycles which mayinclude, but are not limited to, heterocyclic compounds that are5-member or 6-member rings with at least one ring nitrogen. For example,pyridines, imidazoles and related species with at least one five-memberring, bipyridines (e.g., two connected pyridines) and substitutedderivatives were generally found suitable as catalysts for theelectrochemical reduction and/or the photoelectrochemical reduction.Amines that have sulfur or oxygen in the rings may also be suitable forthe reductions. Amines with sulfur or oxygen may include thiazoles oroxazoles. Other aromatic amines (e.g., quinolines, adenine,benzimidazole and 1,10-phenanthroline) may also be effectiveelectrocatalysts.

Before any embodiments of the invention are explained in detail, it isto be understood that the embodiments may not be limited in applicationper the details of the structure or the function as set forth in thefollowing descriptions or illustrated in the figures of the drawing.Different embodiments may be capable of being practiced or carried outin various ways. Also, it is to be understood that the phraseology andterminology used herein is for the purpose of description and should notbe regarded as limiting. The use of terms such as “including,”“comprising,” or “having” and variations thereof herein are generallymeant to encompass the item listed thereafter and equivalents thereof aswell as additional items. Further, unless otherwise noted, technicalterms may be used according to conventional usage.

In the following description of methods, process steps may be carriedout over a range of temperatures (e.g., approximately 10° C. (Celsius)to 50° C.) and a range of pressures (e.g., approximately 1 to 10atmospheres) unless otherwise specified. Numerical ranges recited hereingenerally include all values from the lower value to the upper value(e.g., all possible combinations of numerical values between the lowestvalue and the highest value enumerated are considered expressly stated).For example, if a concentration range or beneficial effect range isstated as 1% to 50%, it is intended that values such as 2% to 40%, 10%to 30%, or 1% to 3%, etc., are expressly enumerated. The above may besimple examples of what is specifically intended.

A use of electrochemical or photoelectrochemical reduction of carbondioxide, tailored with certain electrocatalysts, may produce carbonmonoxide and/or hydrogen gas in a high yield of 0% to about 100%.Relative yields may be controlled by changing the cathode materials,catalysts and various aspects of reaction conditions such as pH andcarbon dioxide flow rate.

The overall reaction for the evolution of synthesis gas from carbondioxide may be represented as follows:

CO₂+H₂O→CO+H₂+O₂

The reduction of the carbon dioxide may be suitably achieved efficientlyin a divided electrochemical or photoelectrochemical cell in which (i) acompartment contains an anode that is an inert counter electrode and(ii) another compartment contains a working cathode electrode and one ormore substituted or unsubstituted aromatic heterocyclic amines. Thecompartments may be separated by a porous glass frit or other ionconducting bridge. Both compartments generally contain an aqueoussolution of an electrolyte. Carbon dioxide gas may be continuouslybubbled through the cathodic electrolyte solution to saturate thesolution.

In the working electrode compartment, carbon dioxide may be continuouslybubbled through the solution. In some embodiments, if the workingelectrode is a conductor, an external bias may be impressed across thecell such that the potential of the working electrode is held constant.In other embodiments, if the working electrode is a p-typesemiconductor, the electrode may be suitably illuminated with light. Anenergy of the light may be matching or greater than a bandgap of thesemiconductor during the electrolysis. Furthermore, either no externalsource of electrical energy may be used or a modest bias (e.g., about500 millivolts) may be applied. The working electrode potential isgenerally held constant relative to a saturated calomel electrode (SCE).The electrical energy for the electrochemical reduction of carbondioxide may come from a normal energy source, including nuclear andalternatives (e.g., hydroelectric, wind, solar power, geothermal, etc.),from a solar cell or other nonfossil fuel source of electricity,provided that the electrical source supply at least 1.6 volts across thecell. Other voltage values may be adjusted depending on the internalresistance of the cell employed.

Advantageously, the carbon dioxide may be obtained from any sources(e.g., an exhaust stream from fossil-fuel burning power or industrialplants, from geothermal or natural gas wells or the atmosphere itself).The carbon dioxide may be obtained from concentrated point sources ofgeneration prior to being released into the atmosphere. For example,high concentration carbon dioxide sources may frequently accompanynatural gas in amounts of 5% to 50%, exist in flue gases of fossil fuel(e.g., coal, natural gas, oil, etc.) burning power plants and nearlypure carbon dioxide may be exhausted of cement factories and fromfermenters used for industrial fermentation of ethanol. Certaingeothermal steams may also contain significant amounts of carbondioxide. The carbon dioxide emissions from varied industries, includinggeothermal wells, may be captured on-site. Separation of the carbondioxide from such exhausts is known. Thus, the capture and use ofexisting atmospheric carbon dioxide in accordance with some embodimentsof the present invention generally allow the carbon dioxide to be arenewable and unlimited source of carbon.

For electrochemical conversions, the carbon dioxide may be readilyreduced in an aqueous medium with a conductive electrode. Faradaicefficiencies have been found high, some reaching about 100%. A mix ofcathode materials may be used to achieve the desired carbon dioxide tohydrogen ratio for the synthesis gas. The mix may include alloys and/oran aggregate of several adjoining materials. The adjoining materials mayform strip patterns, dot patterns, speckles and other multi-surfacedarrangements. For photoelectrochemical conversions, the carbon dioxidemay be readily reduced with a p-type semiconductor electrode, such asp-GaP, p-GaAs, p-InP, p-InN, p-WSe₂, p-CdTe, p-GaInP₂ and p-Si.

The electrochemical/photoelectrochemical reduction of the carbon dioxidegenerally utilizes one or more catalysts in the aqueous solution.Aromatic heterocyclic amines may include, but are not limited to,unsubstituted and substituted pyridines and imidazoles. Substitutedpyridines and imidazoles may include, but are not limited to mono anddisubstituted pyridines and imidazoles. For example, suitable catalystsmay include straight chain or branched chain lower alkyl (e.g., C1-C10)mono and disubstituted compounds such as 2-methylpyridine, 4-tertbutylpyridine, 2,6-dimethylpyridine (2,6-lutidine); bipyridines, such as4,4′-bipyridine; amino-substituted pyridines, such as 4-dimethylaminopyridine; and hydroxyl-substituted pyridines (e.g., 4-hydroxy-pyridine)and substituted or unsubstituted quinoline or isoquinolines. Thecatalysts may also suitably include substituted or unsubstituteddinitrogen heterocyclic amines, such as pyrazine, pyridazine andpyrimidine. Other catalysts generally include azoles, imidazoles,indoles, oxazoles, thiazoles, substituted species and complex multi-ringamines such as adenine, pterin, pteridine, benzimidazole, phenanthrolineand the like.

A variety of heterocycle catalysts may be used. Some of the catalystsmay selectively produce carbon monoxide, such as quinoline. Cathodematerials that work with heterocyclic catalysts generally include Sn,Cu, Cu alloys such as brass or bronze, and stainless steels. Somecathode materials may be combined with other cathode materials moreselective to hydrogen evolution, such as platinum group metals (e.g.,Ir, Os, Pd, Pt, Rh and Ru), to produce a desired carbon monoxide tohydrogen molar ratio at a given potential and pH.

Referring to FIG. 1, a block diagram of a system 100 is shown inaccordance with a preferred embodiment of the present invention. Thesystem (or apparatus) 100 generally comprises a cell (or container) 102,a liquid source 104, a power source 106, a gas source 108 and anextractor 112. An output gas may be presented from the extractor 112.Another output gas may be presented from the cell 102.

The cell 102 may be implemented as a divided cell. The divided cell maybe a divided electrochemical cell and/or a divided photochemical cell.The cell 102 is generally operational to reduce carbon dioxide (CO₂)into one or more products. The reduction generally takes place bybubbling carbon dioxide into an aqueous solution of an electrolyte inthe cell 102. A cathode in the cell 102 may reduce the carbon dioxideand protons into one or more molecules (e.g., carbon monoxide and/orhydrogen) and/or organic compounds.

The cell 102 generally comprises two or more compartments (or chambers)114 a-114 b, a separator (or membrane) 116, an anode 118 and a cathode120. The anode 118 may be disposed in a given compartment (e.g., 114 a).The cathode 120 may be disposed in another compartment (e.g., 114 b) onan opposite side of the separator 116 as the anode 118. An aqueoussolution 122 may fill both compartments 114 a-114 b. A catalyst 124 maybe added to the compartment 114 b containing the cathode 120.

The liquid source 104 may implement a water source. The liquid source104 may be operational to provide pure water to the cell 102.

The power source 106 may implement a variable voltage source. The source106 may be operational to generate an electrical potential between theanode 118 and the cathode 120. The electrical potential may be a DCvoltage.

The gas source 108 may implement a carbon dioxide source. The source 108is generally operational to provide carbon dioxide to the cell 102. Insome embodiments, the carbon dioxide is bubbled directly into thecompartment 114 b containing the cathode 120.

The extractor 112 may implement an oxygen extractor. The extractor 112is generally operational to extract oxygen (e.g., O₂) byproducts createdby the reduction of the carbon dioxide and/or the oxidation of water.The extracted oxygen may be presented through a port 128 of the system100 for subsequent storage and/or consumption by other devices and/orprocesses. Synthesis gases (e.g., carbon monoxide and hydrogen gas)created by the reduction of the carbon dioxide may be extracted from thecell 102 via a port 130.

In the process described, water may be oxidized (or split) into protonsand oxygen at the anode 118 while the carbon dioxide is reduced tocarbon monoxide at the cathode 120. Protons from oxidized water may alsobe reduced to hydrogen gas at the cathode 120. The electrolyte 122 inthe cell 102 may use water as a solvent with any salts that are watersoluble and the catalyst 124. The catalysts 124 may include, but are notlimited to, nitrogen, sulfur and oxygen containing heterocycles.Examples of the heterocyclic compounds may be pyridine, imidazole,pyrrole, thiazole, furan, thiophene and the substituted heterocyclessuch as amino-thiazole and benzimidazole. Cathode materials generallyinclude any conductor. The cathode material may be configured in such away that an appropriate ratio of hydrogen gas and carbon monoxide areproduced at the cathode 120. Generally, the ratio may be one or more(e.g., three) moles of hydrogen gas per mole of carbon monoxide. Anyanode material may be used. The overall process is generally driven bythe power source 106. Combinations of cathodes 120, electrolytes 122,catalysts 124, pH level, flow rate of carbon dioxide to the cell 102,and electric potential from the power source 106 may be used to controlthe reaction products of the cell 102. For example, increasing the flowrate of the carbon dioxide into a 100 milliliter (mL) cell 102 from 5 mLper minute to 10 mL per minute generally increases the yield (e.g., 10%to 20% increase) of carbon monoxide with a corresponding decrease inhydrogen gas.

The process is controlled to get a desired gases by using combinationsof specific cathode materials, catalysts, electrolytes, surfacemorphology of the electrodes, pH levels, electrical potential, flow rateof the carbon dioxide and/or introduction of carbon dioxide relative tothe cathode. Efficiency may be maximized by employing a catalyst/cathodecombination selective for reduction of carbon dioxide to carbon monoxidein conjunction with cathode materials optimized for hydrogen gasevolution. An electrode material may be utilized that does not interactwell with either the heterocyclic catalyst nor the carbon dioxide, buthas a low overpotential for hydrogen evolution. Half cell potentials atthe cathode 120 may range from −0.7 volts to −1.5 volts relative to theSCE, depending on the cathode material used.

Referring to FIG. 2, a table illustrating relative organic productyields for different cathodes are shown. The table generally shows gasevolution in the cell 102 with an aqueous solution of 0.5 M KCl and 10mM pyridine. The carbon dioxide may be bubbled into the cell 102 at therates ranging from 5 ml, per minute to 10 mL per minute. In the presenceof a heterocycle catalyst, combining cathode materials that producemostly carbon monoxide (e.g., C, Cr, Nb, Sn and stainless steel) withmaterials producing mostly hydrogen gas (e.g., Ni, V and platinum groupmetals) a system making a desired ratio of carbon monoxide to hydrogenmay be created. The electrode materials may be in bulk form or presentas particles or nanoparticles loaded on to a substrate such as graphite,carbon fiber or other conductor. Further control over the reaction isgenerally possible by changing a pH level, cell electrical potential andthe flow rate of the carbon dioxide. As illustrated, faradaic yields(FY) for one or both of carbon monoxide and the hydrogen gas may be atleast 25% for several cathode materials.

Cell design and cathode treatment (e.g., surface morphology or surfacetexture) may both affect product yields and current density at thecathode 120. For instance, a divided cell 102 generally has higheryields with a heavily scratched (rough) cathode 120 than an unscratched(smooth) cathode 120. Matte tin generally performs different than brighttin. Maintaining carbon dioxide bubbling only on the cathode side of thedivided cell 102 (e.g., in compartment 114 b) may also increase yields.

Some process embodiments of the present invention for making synthesisgas generally consume a small amount of water (e.g., approximately 1 to3 moles of water) per mole of carbon. Therefore, the processes may be afew thousand times more water efficient than existing productiontechniques.

Referring to FIG. 3, a formula of an aromatic heterocyclic aminecatalyst is shown. The ring structure may be an aromatic 5-memberheterocyclic ring or 6-member heterocyclic ring with at least one ringnitrogen and is optionally substituted at one or more ring positionsother than nitrogen with R. L may be C or N. R1 may be H. R2 may be H ifL is N or R2 is R if L is C. R is an optional substitutent on any ringcarbon and may be independently selected from H, a straight chain orbranched chain lower alkyl, hydroxyl, amino, pyridyl, or two R's takentogether with the ring carbons bonded thereto are a fused six-memberaryl ring and n=0 to 4.

Referring to FIGS. 4-6, formulae of substituted or unsubstitutedaromatic 5-member heterocyclic amines or 6-member heterocyclic aminesare shown. Referring to FIG. 4, R3 may be H. R4, R5, R7 and R8 aregenerally independently H, straight chain or branched chain lower alkyl,hydroxyl, amino, or taken together are a fused six-member aryl ring. R6may be H, straight chain or branched chain lower alkyl, hydroxyl, aminoor pyridyl.

Referring to FIG. 5, one of L1, L2 and L3 may be N, while the other L'smay be C. R9 may be H. If L1 is N, R10 may be H. If L2 is N, R11 may beH. If L3 is N, R12 may be H. If L1, L2 or L3 is C, then R10, R11, R12,R13 and R14 may be independently selected from straight chain orbranched chain lower alkyl, hydroxyl, amino, or pyridyl.

Referring to FIG. 6, R15 and R16 may be H. R17, R18 and R19 aregenerally independently selected from straight chain or branched chainlower alkyl, hydroxyl, amino, or pyridyl.

Suitably, the concentration of aromatic heterocyclic amine catalysts isabout 10 millimolar (mM) to 1 M. Concentrations of the electrolyte maybe about 0.1 M to 1 M. The electrolyte may be suitably a salt, such asKCl, NaNO₃, Na₂SO₄, NaCl, NaF, NaClO₄, KClO₄, K₂SiO₃, or CaCl₂ at aconcentration of about 0.5 M. Other electrolytes may include, but arenot limited to, all group 1 cations (e.g., H, Li, Na, K, Rb and Cs)except Francium (Fr), Ca, ammonium cations, alkylammonium cations andalkyl amines. Additional electrolytes may include, but are not limitedto, all group 17 anions (e.g., F, Cl, Br, T and At), borates,carbonates, nitrates, nitrites, perchlorates, phosphates,polyphosphates, silicates and sulfates. Na generally performs as well asK with regard to best practices, so NaCl may be exchanged with KCl. NaFmay perform about as well as NaCl, so NaF may be exchanged for NaCl orKCl in many cases. Larger anions tend to change the chemistry and favordifferent products. For instance, sulfate may favor polymer or methanolproduction while C1 may favor products such as acetone. The pH of thesolution is generally maintained at about pH 4 to 8, suitably about 4.7to 5.6. Some embodiments of the present invention may be furtherexplained by the following examples, which should not be construed byway of limiting the scope of the invention.

Example 1 General Electrochemical Methods

Chemicals and materials. All chemicals used were >98% purity and used asreceived from the vendor (e.g., Aldrich), without further purification.Either deionized or high purity water (Nanopure, Barnstead) was used toprepare the aqueous electrolyte solutions.

Electrochemical system. The electrochemical system was composed of astandard two-compartment electrolysis cell 102 to separate the anode 118and cathode 120 reactions. The compartments were separated by a porousglass frit or other ion conducting bridge 116. The electrolytes 122 wereused at concentrations of 0.1 M to 1 M, with 0.5 M being a typicalconcentration. A concentration of between about 1 mM to 1M of thecatalyst 124 was used. The particular electrolyte 122 and particularcatalyst 124 of each given test were generally selected based upon whatproduct or products were being created.

Referring to FIG. 7, a flow diagram of an example method 140 used in theelectrochemical examples is shown. The method (or process) 140 generallycomprises a step (or block) 142, a step (or block) 144, a step (orblock) 146, a step (or block) 148 and a step (or block) 150. The method140 may be implemented using the system 100.

In the step 142, the electrodes 118 and 120 may be activated whereappropriate. Bubbling of the carbon dioxide into the cell 102 may beperformed in the step 144. Electrolysis of the carbon dioxide intovarious products may occur during step 146. In the step 148, theproducts may be separated from the electrolyte. Analysis of thereduction products may be performed in the step 150.

The working electrode was of a known area. All potentials were measuredwith respect to a saturated calomel reference electrode (Accumet).Before and during all electrolysis, carbon dioxide (Airgas) wascontinuously bubbled through the electrolyte to saturate the solution.The resulting pH of the solution was maintained at about pH 4 to pH 8with a suitable range depending on what product or products were beingmade. For example, under constant carbon dioxide bubbling, the pH levelsof 10 mM solutions of 4-hydroxy pyridine, pyridine and 4-tertbutylpyridine were 4.7, 5.28 and 5.55, respectively.

Example 2 General Photoelectrochemical Methods

Chemicals and materials. All chemicals used were analytical grade orhigher. Either deionized or high purity water (Nanopure, Barnstead) wasused to prepare the aqueous electrolyte solutions.

Photoelectrochemical system. The photoelectrochemical system wascomposed of a Pyrex three-necked flask containing 0.5 M KCl assupporting electrolyte and a 1 mM to 1 M catalyst (e.g., 10 mM pyridineor pyridine derivative). The photocathode was a single crystal p-typesemiconductor etched for approximately 1 to 2 minutes in a bath ofconcentrated HNO₃:HCl, 2:1 v/v prior to use. An ohmic contact was madeto the back of the freshly etched crystal using an indium/zinc (2 wt. %Zn) solder. The contact was connected to an external lead withconducting silver epoxy (Epoxy Technology H31) covered in glass tubingand insulated using an epoxy cement (Loctite 0151 Hysol) to expose onlythe front face of the semiconductor to solution. All potentials werereferenced against a saturated calomel electrode (Accumet). The threeelectrode assembly was completed with a carbon rod counter electrode tominimize the reoxidation of reduced carbon dioxide products. During allelectrolysis, carbon dioxide gas (Airgas) was continuously bubbledthrough the electrolyte to saturate the solution. The resulting pH ofthe solution was maintained at about pH 4 to 8 (e.g., pH 5.2).

Referring to FIG. 8, a flow diagram of an example method 160 used in thephotochemical examples is shown. The method (or process) 160 generallycomprises a step (or block) 162, a step (or block) 164, a step (orblock) 166, a step (or block) 168 and a step (or block) 170. The method160 may be implemented using the system 100.

In the step 162, the photoelectrode may be activated. Bubbling of thecarbon dioxide into the cell 102 may be performed in the step 164.Electrolysis of the carbon dioxide into various products may occurduring step 166. In the step 168, the products may be separated from theelectrolyte. Analysis of the reduction products may be performed in thestep 170.

Light sources. Four different light sources were used for theillumination of the p-type semiconductor electrode. For initialelectrolysis experiments, a Hg—Xe arc lamp (USHIO UXM 200H) was used ina lamp housing (PTI Model A-1010) and powered by a PTI LTS-200 powersupply. Similarly, a Xe arc lamp (USHIO UXL 151H) was used in the samehousing in conjunction with a PTI monochromator to illuminate theelectrode at various specific wavelengths.

A fiber optic spectrometer (Ocean Optics 52000) or a siliconphotodetector (Newport 818-SL silicon detector) was used to measure therelative resulting power emitted through the monochromator. The flatbandpotential was obtained by measurements of the open circuit photovoltageduring various irradiation intensities using the 200 watt (W) Hg—Xe lamp(3 W/cm²-23 W/cm²). The photovoltage was observed to saturate atintensities above approximately 6 W/cm².

For quantum yield determinations, electrolysis was performed underillumination by two different light-emitting diodes (LEDs). A blue LED(Luxeon V Dental Blue, Future Electronics) with a luminous output of 500milliwatt (mW)+/−50 mW at 465 nanometers (nm) and a 20 nm full width athalf maximum (FWHM) was driven at to a maximum rated current of 700 mAusing a Xitanium Driver (Advance Transformer Company). A Fraencollimating lens (Future Electronics) was used to direct the outputlight. The resultant power density that reached the window of thephotoelectrochemical cell was determined to be 42 mW/cm², measured usinga Scientech 364 thermopile power meter and silicon photodetector. Themeasured power density was assumed to be greater than the actual powerdensity observed at the semiconductor face due to luminous intensityloss through the solution layer between the wall of thephotoelectrochemical cell and the electrode.

Example 3 Analysis of Products of Electrolysis

Electrochemical experiments were generally performed using a CHInstruments potentiostat or a DC power supply with current logger to runbulk electrolysis experiments. The CH Instruments potentiostat wasgenerally used for cyclic voltammetry. Electrolysis was run underpotentiostatic conditions from approximately 6 hours to 30 hours until arelatively similar amount of charge was passed for each run.

Gas Chromatography and Detection of Gaseous Products. The gas productsevolved during electrolysis were analyzed using a Quest Technologies COdetector and a QMS300 quadrupole mass spectrometer. For dissolvedproducts in the aqueous phase, the removal of the supporting electrolytesalt was first achieved with an Amberlite IRN-150 ion exchange resin(cleaned prior to use to ensure no organic artifacts by stirring in a0.1% v/v aqueous solution of Triton X-100, reduced (Aldrich), filteredand rinsed with a copious amount of water, and vacuum dried below themaximum temperature of the resin (approximately 60° C.) before thesample was directly injected into the GC which housed a DB-Wax column(Agilent Technologies, 60 m, 1 micrometer (μm) film thickness).Approximately 1 gram of resin was used to remove the salt from 1milliliter (mL) of the sample. The injector temperature was held at 200°C., the oven temperature maintained at 120° C., and the detectortemperature at 200° C.

Carbon dioxide may be efficiently converted to value-added gases, usingeither a minimum of electricity (that could be generated from analternate energy source) or directly using visible light. Some processesdescribed above may generate high energy density fuels that are notfossil-based as well as being chemical feedstock that are not fossil orbiologically based. Moreover, the catalysts for the processes may besubstituents-sensitive and provide for selectivity of the value-addedgases.

By way of example, a fixed cathode may be used in an electrochemicalsystem where the electrolyte and/or catalyst are altered to change thegas mix. In a modular electrochemical system, the cathodes may beswapped out with different materials to change the gas mix. In aphotoelectrochemical system, the anode and/or cathode may use differentphotovoltaic materials to change the gas mix.

Some embodiments of the present invention generally provide for newcathode combinations for simultaneous evolution of carbon monoxide andhydrogen gas using carbon dioxide and water as feedstock. Specificcombinations of cathode materials, electrolytes, catalysts, pH levelsand/or electrical potentials may be established that optimize the carbondioxide conversion to carbon monoxide while also optimizing hydrogen gasevolution. Choice of specific configurations of heterocyclic aminecatalysts with engineered functional groups may be utilized in thesystem 100. Process conditions described above may facilitate long life(e.g., improved stability), electrode and cell cycling and productrecovery.

Various process conditions disclosed above, including electrolytechoice, cell voltage, and manner in which the carbon dioxide is bubbled,generally improve control of the reaction so that precise molar ratioswithin synthesis gas may be maintained with little or no byproducts.Greater control over the reaction generally opens the possibility forcommercial systems that are modular and adaptable to make differentgases. The new materials and process conditions combinations generallyhave high faradaic efficiency and relatively low cell potentials, whichallows an energy efficient cell to be constructed.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the scope of the invention.

1. A method for electrochemical production of synthesis gas from carbondioxide, comprising the steps of: (A) bubbling said carbon dioxide intoa solution of an electrolyte and a catalyst in a divided electrochemicalcell, wherein (i) said divided electrochemical cell comprises an anodein a first cell compartment and a cathode in a second cell compartment,(ii) said cathode reducing said carbon dioxide into a plurality ofcomponents; (B) establishing a molar ratio of said components in saidsynthesis gas by adjusting at least one of (i) a cathode material and(ii) a surface morphology of said cathode; and (C) separating saidsynthesis gas from said solution.
 2. The method according to claim 1,wherein said components comprises carbon monoxide and hydrogen.
 3. Themethod according to claim 2, wherein said molar ratio comprises at leastone mole of said hydrogen per mole of said carbon monoxide.
 4. Themethod according to claim 1, wherein said cathode material is at leastone of Al, Au, Ag, C, Cd, Co, Cr, Cu, Cu alloys, Ga, Hg, In, Ir, Mo, Nb,Ni, Ni alloys, Ni—Fe alloys, Os, Pd, Pt, Rh, Ru, Sn, Sn alloys, Ti, V,W, Zn, elgiloy, Nichrome, austenitic steel, duplex steel, ferriticsteel, martensitic steel, stainless steel, degenerately doped p-Si,degenerately doped p-Si:As and degenerately doped p-Si:B.
 5. The methodaccording to claim 1, wherein (i) said cathode comprises a plurality ofparticles of metal loaded onto a substrate and (ii) said substrate isconductive.
 6. The method according to claim 1, wherein said surfacemorphology of said cathode comprises a smooth surface.
 7. The methodaccording to claim 1, wherein said surface morphology of said cathodecomprises a rough surface.
 8. A method for electrochemical production ofsynthesis gas from carbon dioxide, comprising the steps of: (A) bubblingsaid carbon dioxide into a solution of an electrolyte and a catalyst ina divided electrochemical cell, wherein (i) said divided electrochemicalcell comprises an anode in a first cell compartment and a cathode in asecond cell compartment, (ii) said cathode reducing said carbon dioxideinto a plurality of components; (B) establishing a molar ratio of saidcomponents in said synthesis gas by adjusting one or more of (i) saidelectrolyte, and (ii) said catalyst; and (C) separating said synthesisgas from said solution.
 9. The method according to claim 8, wherein saidcomponents comprises carbon monoxide and hydrogen.
 10. The methodaccording to claim 8, wherein said electrolyte is at least one ofNa₂SO₄, KCl, NaNO₃, NaCl, NaF, NaClO₄, KClO₄, K₂SiO₃, CaCl₂, a H cation,a Li cation, a Na cation, a K cation, a Rb cation, a Cs cation, a Cacation, an ammonium cation, an alkylammonium cation, a F anion, a Clanion, a Br anion, an I anion, an At anion, an alkyl amine, borates,carbonates, nitrites, nitrates, phosphates, polyphosphates,perchlorates, silicates, sulfates, and a tetraalkyl ammonium salt. 11.The method according to claim 8, wherein said catalyst is one or more ofadenine, amines containing sulfur, amines containing oxygen, azoles,benzimidazole, bipyridines, furan, imidazoles, imidazole related specieswith at least one five-member ring, indoles, methylimidazole, oxazoles,phenanthroline, pterin, pteridine, pyridines, pyridine related specieswith at least one six-member ring, pyrrole, quinoline and thiazoles. 12.The method according to claim 8, wherein a faradaic yield of hydrogen insaid synthesis gas is at least 25 percent.
 13. The method according toclaim 8, wherein a faradaic yield of carbon monoxide in said synthesisgas is at least 25 percent.
 14. The method according to claim 8, whereinsaid establishing of said molar ratio includes adjusting at least one of(i) a cathode material and (ii) a surface morphology of said cathode.15. A method for electrochemical production of synthesis gas from carbondioxide, comprising the steps of: (A) bubbling said carbon dioxide intoa solution of an electrolyte and a catalyst in a divided electrochemicalcell, wherein (i) said divided electrochemical cell comprises an anodein a first cell compartment and a cathode in a second cell compartment,(ii) said cathode reducing said carbon dioxide into a plurality ofcomponents; (B) establishing a molar ratio of said components in saidsynthesis gas by adjusting one or more of (i) a pH level and (ii) a flowrate of said carbon dioxide; and (C) separating said synthesis gas fromsaid solution.
 16. The method according to claim 15, wherein saidcomponents comprises carbon monoxide and hydrogen.
 17. The methodaccording to claim 16, wherein said molar ratio comprises at least onemole of said hydrogen per mole of said carbon monoxide.
 18. The methodaccording to claim 15, wherein said pH level ranges from approximately 4to approximately
 8. 19. The method according to claim 15, wherein anelectrical potential ranges from approximately −0.7 volts toapproximately −1.5 volts.
 20. The method according to claim 15, whereinsaid establishing of said molar ratio includes adjusting at least one of(i) a cathode material and (ii) a surface morphology of said cathode.